Hi Paul,
As per my experience, the spiral hook allows you to mix a bigger amount of dough than the C-shaped one using the same mixer (same bowl size and same mechanical power) : with a fully loaded bowl, I noticed that the C hook tends to drag almost of the dough and then to compress it against the bowl, causing "hard steps" during the rotation of the transmission shaft which stops-&-goes periodically (and with probably bad consequences against the motor and gearing) ... while the spiral hook seems to bear such a job better, and allows the mechanic to run regularly.
At all events, the planetary and spiral mixers knead the dough intensely and forces it to "heat up" very quickly. For the pizza dough I don't know if this is a real problem, but for bread yeast leavened dough pastries this is not usually optimal.
For me the best machines for kneading are the diving arms mixers, especially those made by the Artofex Company (obviously !). You can watch a video of this kind of mixer here : myplot.org/vids/dough.html
